Evaluation Methods for Improving Power System Stability of Controlled Commutation Converters

Abstract

This invention provides a method for evaluating the power system stability improvement capability of controllable commutated converters (CLCCs), addressing the lack of quantitative evaluation of the stability effects of novel CLCC converters in existing technologies. Specifically, this method aims to comprehensively evaluate the effects of CLCCs in suppressing commutation failures, mitigating voltage fluctuations, and enhancing system disturbance immunity by defining the "Effective System Strength Ratio (SCESSR)" for single-circuit DC systems and the "Effective Cooperative Strength Ratio (MCESSR)" for multi-circuit DC systems, combined with simulation verification. This provides a scientific basis for power grid planning, operation control, and fault diagnosis, improving the safety and stability of AC / DC interconnected power grids in scenarios with a high proportion of renewable energy.

Description

Technical Field

[0001] This invention relates to the field of high voltage direct current (HVDC) transmission technology, and in particular to a multi-HVDC coordinated control method in HVDC and ultra-high voltage direct current (UHVDC) transmission systems containing controllable phase-commutation valves. Background Technology

[0002] To address the issue of continuous blocking in multi-infeed DC systems and fundamentally eliminate the long-term threat posed by commutation failure, thereby improving system stability, the State Grid Corporation of China, in collaboration with relevant research institutes and industry units, has continuously improved the AC fault ride-through capability and reactive power voltage support capability of DC transmission systems to reduce the risk of commutation failure. In 2020, they proposed the Controlled Commutated Converter (CLCC) solution. This converter fully utilizes the technical advantages of thyristor devices (large capacity, low loss) and IGBTs (strong turn-off capability), combining the strengths of both. It inherits the economic advantages of traditional LCC converters while fundamentally avoiding commutation failure and improving grid stability. However, a clear evaluation method is lacking regarding the stability improvement capability of CLCC in power systems. Summary of the Invention

[0003] This invention proposes an evaluation method for the power system stability improvement capability of controllable commutated converters (CLCCs), addressing the lack of quantitative evaluation of the stability effects of novel CLCC converters in existing technologies. Specifically, this method aims to comprehensively evaluate the effects of CLCCs in suppressing commutation failures, mitigating voltage fluctuations, and enhancing system disturbance immunity by defining the "Effective System Strength Ratio (SCESSR)" for single-circuit DC systems and the "Effective Cooperative Strength Ratio (MCESSR)" for multi-circuit DC systems, combined with simulation verification. This provides a scientific basis for power grid planning, operation control, and fault diagnosis, improving the safety and stability of AC / DC interconnected power grids in scenarios with a high proportion of renewable energy.

[0004] The objective of this invention is achieved through the following technical solution:

[0005] A method for evaluating the ability of a controllable commutator to improve power system stability, the method comprising:

[0006] S100. Determine whether the DC system in the power grid is a single-circuit DC system or a multi-circuit DC system;

[0007] S200. Build a single-circuit DC power system model, propose an effective system strength ratio, and evaluate the stability improvement effect in the single-circuit DC system;

[0008] S300. Build a multi-circuit DC power system model, propose an effective coordination strength ratio, and evaluate the stability improvement effect in the multi-circuit DC system;

[0009] S400. Use simulation tools to verify the accuracy of the stability improvement effect.

[0010] Preferably, S100 includes:

[0011] Based on topology analysis, the nodes of the converter stations are obtained. If there are only 2 converter stations in the power grid and the DC lines are connected point-to-point, it is determined to be a single-circuit DC system. If there are ≥3 converter stations or multiple independent converter units are configured in the same converter station, it is determined to be a multi-circuit DC system.

[0012] Preferably, S100 further includes:

[0013] Monitor real-time data from each converter station, extract the DC network topology from the system or design documents, and establish a hybrid power flow model that includes DC. If all DC power is concentrated on a single line, it is determined to be a single-circuit DC system; if the power is distributed across multiple lines and has complementary characteristics, it is determined to be a multi-circuit DC system.

[0014] Preferably, S100 further includes:

[0015] If an N-1 fault is triggered by simulation or historical data, and the power increase of the remaining DC branch is greater than or equal to 80% of the capacity of the faulty branch, it is determined to be a multi-circuit DC system; otherwise, it is determined to be a single-circuit DC system.

[0016] Preferably, S100 further includes:

[0017] Set a weight threshold. If the total score of multiple features is ≥0.7, it is determined to be a multiple DC system; otherwise, it is determined to be a single DC system.

[0018] Preferably, the effective system strength ratio (SCESSR) is calculated using the following formula:

[0019] ,

[0020] in, This refers to the short-circuit capacity of the receiving-end AC bus in a single-loop DC system. To enhance the dynamic reactive power compensation capability of the converter station, denoted as the rated transmission power of the DC system, and k is the dynamic reactive power weighting coefficient.

[0021] Preferably, the dynamic reactive power weighting coefficient k is set to 1.0 if the converter adopts fast voltage control, and to 0.5 if only slow reactive power compensation is provided.

[0022] Preferably, the effective system strength ratio SCESSR > 1.5: the system strength is sufficient, the voltage stability is high, and the fault recovery capability is strong;

[0023] 1.0 ≤ SCESSR ≤ 1.5: The system is at critical strength and requires dynamic control measures to maintain stability;

[0024] SCESSR < 1.0: The system is not strong enough and there is a risk of voltage collapse or power transfer limitation.

[0025] Preferably, the effective synergy strength ratio (MCESSR) is calculated using the following formula:

[0026] ,

[0027] in, Let the short-circuit capacity of the AC bus at the connection point of the i-th converter station be . Let i be the dynamic reactive power compensation capability of the i-th converter station. Let be the reactive power contribution efficiency coefficient of the i-th converter station. Let i be the rated transmission power of the i loops. For redundant power coordination, This represents the system's transferable redundant power. It is a cooperating factor.

[0028] Preferably, the effective coordination strength ratio MCESSR > 2.0: the system has high coordination strength, sufficient dynamic support between multiple loops, and strong fault tolerance capability;

[0029] 1.2 ≤ MCESSR ≤ 2.0: The coordination strength is moderate, and stability needs to be maintained by optimizing the control strategy;

[0030] MCESSR < 1.2: Insufficient coordination strength, with the risk of cascading failures or power blocking.

[0031] Preferably, this invention proposes a method for evaluating the ability of a controllable commutation converter to improve power system stability. First, a DC transmission system model is established using the PSCAD / EMTDC simulation tool. Then, a short-circuit fault (such as a three-phase fault) is set in a single-circuit branch, and the stability improvement strength is verified by observing the voltage recovery time and power recovery time. In a multi-circuit branch, multiple simultaneous faults (such as DC blocking of two circuits) are set, and the stability improvement strength is verified by observing the power increase and voltage stability of the remaining circuits.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. It can provide a method for evaluating the ability of CLCC to improve the stability of power systems.

[0034] 2. The two proposed indicators, "Effective System Strength Ratio (SCESSR)" for single-circuit DC systems and "Effective Coordination Strength Ratio (MCESSR)" for multi-circuit DC systems, can both improve the assessment of power system stability and, to some extent, help with power system planning. Attached Figure Description

[0035] Figure 1 This is the overall model for CLCC simulation in one embodiment of the present invention;

[0036] Figure 2 This is a specific flowchart of one embodiment of the present invention;

[0037] Figure 3 This is a flowchart of an evaluation method for improving the stability of a power system using a controllable commutator, according to one embodiment of the present invention.

[0038] Figure 4 This is a hybrid power flow model containing DC in one embodiment of the present invention;

[0039] Figure 5 This is a schematic diagram of the power enhancement of the remaining DC branch in one embodiment of the present invention;

[0040] Figure 6 This is a single-circuit DC power system model in one embodiment of the present invention;

[0041] Figure 7 This is a multi-circuit DC power system model in one embodiment of the present invention;

[0042] Figure 8 This is a schematic diagram of the conduction process of a controllable commutation valve in one embodiment of the present invention. Detailed Implementation

[0043] The following will refer to the appendices in the embodiments of the present invention. Figures 1 to 8 The technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0044] In the description of this invention, it should be noted that the terms "upper", "lower", "left", "right", "top / bottom", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0045] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installed," "equipped with," "sleeved / connected," "connected," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0046] This invention provides the following technical solutions:

[0047] A method for evaluating the ability of a controllable commutator to improve power system stability, the method comprising:

[0048] S100. Determine whether the DC system in the power grid is a single-circuit DC system or a multi-circuit DC system;

[0049] S200. Build a single-circuit DC power system model, propose an effective system strength ratio, and evaluate the stability improvement effect in the single-circuit DC system;

[0050] S300. Build a multi-circuit DC power system model, propose an effective coordination strength ratio, and evaluate the stability improvement effect in the multi-circuit DC system;

[0051] S400. Use simulation tools to verify the accuracy of the stability improvement effect.

[0052] In one embodiment, the CLCC converter model includes multiple controllable commutation valves, such as Figure 1 As shown, a three-phase AC power supply is connected to a converter bridge via a Y-connection. The converter bridge consists of six controllable phase-commutation valves (V1, V2, V3, V4, V5, V6). The controllable phase-commutation valves are turned on and off in a certain triggering sequence to realize the conversion of AC power to DC power.

[0053] Each controllable commutator valve includes a main branch and an auxiliary branch. The main branch includes high-voltage, high-current thyristor valves and low-voltage, high-current IGBT valves, responsible for the main power transmission tasks. The auxiliary branch includes high-voltage, low-current auxiliary valves for auxiliary control and protection functions.

[0054] Under normal operating conditions, the thyristor valves of the main branch circuit turn on and off according to a predetermined triggering sequence, achieving efficient power transmission. In the event of a system failure or the need for special control, the auxiliary branch circuit can respond quickly, providing additional control and protection functions.

[0055] Furthermore, the process is as follows Figure 8As shown, in the initial state, V1 and V2 are turned on, and the current flows from phase A of the power supply into the load through V1, and then returns to point N of the power supply through V2.

[0056] When the voltage of phase B of the power supply is higher than that of phase A, the commutation process begins. V3 and V2 gradually conduct, and the current begins to flow from phase A to phase B. During the commutation process, the circuit can be simplified into an equivalent circuit for analysis, namely the commutation equivalent circuit.

[0057] After commutation, the current flows from phase B of the power supply into the load through V3, and then returns to point N of the power supply through V2.

[0058] Figure 8 The timing diagram below shows the conduction status of the six controllable commutator valves (V1 to V6) in different time intervals.

[0059] In another embodiment, S100 includes:

[0060] S101: Based on topology analysis, obtain the nodes of the converter station. If there are only 2 converter stations in the power grid (1 at the sending end and 1 at the receiving end), and the DC lines are connected point-to-point, it is initially determined to be a single-circuit DC system (characteristic F1); if there are ≥3 converter stations or multiple independent converter units are configured in the same converter station (such as two sets of 12-pulse converters), it is determined to be a multi-circuit DC system (characteristic F2).

[0061] In another embodiment, S100 further includes:

[0062] S102: Monitor real-time data of each converter station, extract DC network topology from SCADA / EMS system or design documents, including converter station location, DC line connection relationship and equipment parameters, obtain power, voltage and current monitoring values ​​of DC lines, and voltage and phase angle information of AC nodes.

[0063] S102-1: Establish a hybrid power flow model including DC, such as Figure 4 As shown, power is transmitted between the sending-end converter station and the receiving-end converter station via a ±500kV DC line. The sending-end converter station is preferably a VSC (Voltage Source Converter), LCC (Linear Commutated Converter), or CLCC (Controllable Commutated Converter), with a transmitting power P. send The dynamic reactive power Q is -1000MW. dyn The received power is ±200 Mvar. The preferred receiving converter station is VSC, LCC, or CLCC, with a receiving power P. recv The dynamic reactive power Q is 980MW. dyn It is ±200Mvar.

[0064] The sending-end converter station and the receiving-end converter station are each connected to their respective AC busbars via 230kV AC sides. The short-circuit capacity S of the sending-end AC busbar... sc The voltage rating is 5000MVA, and the per-unit voltage value V is 1.02pu. The short-circuit capacity S of the receiving-end AC bus is... sc For the same 5000MVA, the per-unit voltage V is also 1.02pu.

[0065] The sending-end AC bus connects to an AC system that includes generators, loads, and networks, and ultimately to the AC power grid. The receiving-end AC bus connects to an AC system that includes loads and a STATCOM (Static Synchronous Compensator), and also ultimately to the AC power grid.

[0066] Single-circuit system model: The DC branch is equivalent to a PQ node pair, satisfying: , where P send P represents the output power of the sending-end converter station. recv P represents the power absorbed by the receiving-end converter station. loss This indicates the loss current.

[0067] Multi-cycle system model: embedding multi-cycle coordination equations, such as , where P dc,i P represents the power of each converter station node; total This indicates the total power.

[0068] S102-2: Based on power characteristic analysis, if all DC power is concentrated on a single line (fluctuation range ≤ rated value ±5%), it is determined to be a single-circuit DC system (characteristic F3); if the power is distributed across multiple lines and there are complementary characteristics (such as one line automatically increasing capacity when it is fully loaded), it is determined to be a multi-circuit DC system (characteristic 4).

[0069] Example as follows:

[0070] Single-circuit DC system: Gezhouba-Nanqiao DC project

[0071] The total transmission capacity of 3000MW relies entirely on a single line. Historical operating data shows that power fluctuations have consistently been kept within ±150MW (±5%).

[0072] Multi-circuit DC system: Jinsha River Hydropower Transmission Project (Xiangjiaba / Xiluodu to Zhejiang DC transmission group)

[0073] The total power of 6400MW is distributed across two independent DC lines (each with a rated capacity of 3200MW).

[0074] The two lines show a significant negative correlation: when the Xiluodu DC is fully loaded, the Xiangjiaba DC automatically reduces its load; and vice versa (as shown in the table below).

[0075]

[0076] In another embodiment, S100 further includes:

[0077] S103: Based on fault response analysis, since a single-circuit DC system can only detect a single power / voltage control command (such as constant power mode), while a multi-circuit system can detect multiple objective optimization commands (such as power balancing and voltage droop control), an N-1 fault is triggered by simulation or historical data (i.e., assuming a branch fails and goes out of operation). If the power increase of the remaining DC branches is ≥ 80% of the capacity of the faulty branch, it is determined to be a multi-circuit DC system (characteristic F5).

[0078] Specifically, when an N-1 fault occurs in a DC branch, the power increase of the remaining branches must meet the following requirements:

[0079] ∑ΔP boost,i ≥0.8⋅P fault

[0080] Wherein, ΔP boost,i P represents the power increase (MW) of the i-th remaining branch; fault The rated capacity (MW) of the faulty branch. If the above conditions are met, the system is a multi-circuit DC system; otherwise, it is a single-circuit system.

[0081] Example: System topology such as Figure 5 As shown, the converter stations are: A (sending end), B, and C (receiving end); the DC branch is: A→B (primary power P). dc,A→B =1000 MW), A→C (original power P) dc,A→C =800 MW), B→C (original power P) dc,B→C =600 MW); Redundancy power: Each branch can be overloaded by up to 20% (i.e., γ=1.2).

[0082] Fault scenario: Assume a fault occurs in branch A→B (P fault =1000MW), the power boosting capacity of the remaining branches A→C and B→C needs to be calculated.

[0083] Calculation process:

[0084] Step 1: Maximum allowable power of remaining branches

[0085] Branch A → C: P max , A→C =γ⋅P dc,A→C =1.2 × 800 = 960MW

[0086] Branch road B→C: P max,B→C =γ⋅P dc , B→C=1.2 × 600 = 720MW

[0087] Step 2: Actual power increase

[0088] The original power of branch A→C is 800 MW, which can be increased by: ΔP boost , A→C =960−800=160MW

[0089] The original power of branch line B→C is 600 MW, which can be increased by: ΔP boost,B→C =720−600=120MW

[0090] Step 3: Total power increase ∑ΔP boost =160+120=280MW

[0091] Step 4: Verify the decision criteria

[0092] 0.8⋅P fault =0.8×1000=800MW

[0093] Since 280 MW < 800 MW, the determination criteria are not met, so it is determined to be a single-circuit DC system.

[0094] In another embodiment, S100 further includes:

[0095] S104: Set weight thresholds (e.g., topology feature A weight 40%, power feature B weight 30%, fault response C weight 30%). If the total score of multiple features M = 0.4A + 0.3B + 0.3C ≥ 0.7, it is determined to be a multi-circuit DC system; otherwise, it is a single-circuit DC system.

[0096] In another embodiment, S200 includes:

[0097] S201: Using the simulation software PSCAD, a detailed single-circuit DC power system model was built based on the CLCC topology, such as... Figure 6 As shown, parameters are adjusted to make the operating state of the controllable commutator converter consistent with reality. For example: at the sending end, the AC grid provides 230kV, 50H... z The AC power is input to the sending-end converter station. The CLCC converter in the sending-end converter station converts the AC power into ±500kV DC power, and transmits it to the distant receiving-end converter station with a power of 1000MW through the high-voltage DC transmission line.

[0098] At the receiving end, the CLCC converter in the receiving-end converter station converts the ±500kV DC power back into 230kV, 50Hz AC power. The converted AC power is then connected to the receiving-end AC power grid via transmission lines for user use.

[0099] S202: To calculate the improvement capability of a single-loop DC system's stability, the Effective System Strength Ratio (SCESSR) is proposed. This ratio quantifies the matching relationship between AC system strength and DC system operating capability, and is defined as follows:

[0100]

[0101] Short-circuit capacity (MVA) of the receiving-end AC bus in a single-loop DC system;

[0102] The dynamic reactive power compensation capability (Mvar) of the converter station, including the reactive power regulation range of STATCOM, SVG, or the converter itself;

[0103] Rated transmission power of DC system (MW);

[0104] k: Dynamic reactive power weighting coefficient (usually taken as 0.5~1.0, reflecting the contribution efficiency of reactive power support to system strength).

[0105] The physical meaning of the effective system strength ratio (SCESSR) is as follows:

[0106] SCESSR > 1.5: The system has sufficient strength, high voltage stability, and strong fault recovery capability;

[0107] 1.0 ≤ SCESSR ≤ 1.5: The system is at critical strength and requires dynamic control measures to maintain stability;

[0108] SCESSR < 1.0: The system is not strong enough and there is a risk of voltage collapse or power transfer limitation.

[0109] Furthermore, the system has sufficient strength, resulting in high voltage stability.

[0110] The AC bus voltage U is determined by the injected current I. Decide:

[0111]

[0112] High system strength means S sc The voltage fluctuation is smaller for the same current change (ΔU∝1 / S). sc ).

[0113] Sufficient system strength → Strong fault recovery capability:

[0114] 1. Fault current support:

[0115] 1) Strong system: Short-circuit current I sc = U n / Zsys I sc Larger sizes result in more sensitive protection devices with 20% to 40% shorter response times. Among them, U... n This is the rated voltage.

[0116] 2) Weak system: I sc Minor issues may include protection failures or delayed fault clearing (e.g., a wind farm grid disconnection incident was delayed by 200ms due to insufficient fault current).

[0117] 2. Transient kinetic energy reserve:

[0118] 1) System kinetic energy E k =1 / 2Jω 2 (J: Equivalent inertia)

[0119] 2) High SCR systems are usually accompanied by large-capacity synchronous machines → large J → low frequency change rate df / dt during faults (as shown in the table below).

[0120]

[0121] 3. Self-healing mechanism activated:

[0122] 1) Strong system: Fast voltage recovery → Trigger SVG / SVC dynamic reactive power compensation (response time <50ms).

[0123] 2) Weak system: The voltage remains low → Reactive equipment cannot start (requires slow synchronous condenser).

[0124] The present invention proposes an Effective System Strength Ratio (SCESSR) to quantify the matching relationship between AC system strength and DC system operating capability, and to verify stability. It also provides a quantitative standard for evaluating the voltage stability of a single-circuit DC system, helping to determine whether dynamic control measures are needed to maintain stability.

[0125] Further:

[0126] S202-1: Perform data acquisition to obtain the short-circuit capacity of the receiving-end AC bus. (Through short-circuit calculation or actual measurement); Extract the dynamic reactive power compensation capacity of the converter station. (e.g., VSC reactive power regulation range ±200Mvar); Determine the DC rated power. (e.g., 1000MW).

[0127] S202-2: Calibrate the k value based on the effect of the control strategy: If the converter adopts fast voltage control (response time < 50ms), take k = 1.0; if it only has slow reactive power compensation (response time > 200ms), take k = 0.5.

[0128] S202-3: Calculate SCESSR;

[0129] S202-4: Conduct stability verification by simulating an AC fault (such as a three-phase short circuit) in PSCAD / EMTDC and observing the following indicators: voltage drop during the fault (target: ≥70% per unit value) and power recovery time (target: <200ms).

[0130] For example:

[0131] 1. SCESSR > 1.5: The system strength is sufficient.

[0132] Parameter setting: Short-circuit capacity of AC bus at receiving end S sc =5000 MVA; Converter station dynamic reactive power compensation capacity Q dyn =300Mvar; Dynamic reactive power weighting coefficient k=1.0 (fast voltage control, response time <50ms); DC system rated power P dc =1000MW.

[0133] = (5000 + 1.0 × 300) / 1000 = 5.3

[0134] SCESSR = 5.3 > 1.5, indicating sufficient system strength, high voltage stability, and strong fault recovery capability.

[0135] 2. 1.0 ≤ SCESSR ≤ 1.5: System strength criticality

[0136] Parameter setting: Short-circuit capacity of AC bus at receiving end S sc =2000MVA; Converter station dynamic reactive power compensation capacity Q dyn =200Mvar; Dynamic reactive power weighting coefficient k=0.8 (medium-speed reactive power compensation, response time≈100ms); DC system rated power P dc =2000MW.

[0137] =1.08

[0138] SCESSR = 1.08 ∈ [1.0, 1.5] indicates that the system strength is critical and requires dynamic control measures to maintain stability.

[0139] 3. SCESSR < 1.0: Insufficient system strength

[0140] Parameter setting: Short-circuit capacity of AC bus at receiving end S sc =800MVA; Converter station dynamic reactive power compensation capacity Q dyn=100Mvar; Dynamic reactive power weighting coefficient k=0.5 (slow reactive power compensation, response time>200ms); DC system rated power P dc =1000MW.

[0141] =0.85

[0142] SCESSR = 0.85 < 1.0, indicating insufficient system strength and a risk of voltage collapse or limited power transmission.

[0143] In another embodiment, S300 includes:

[0144] S301: Using the simulation software PSCAD, a detailed multi-circuit DC power system model was built based on the CLCC topology, such as... Figure 7 As shown, parameters are adjusted to ensure the controllable commutator's operating state matches reality. For example: Converter station A acts as the sending end, with a rated power of 1000MW and a dynamic reactive power capacity of ±200Mvar. Converter station A is connected to converter station B via DC line 1 (±500kV).

[0145] Converter station B, acting as the receiving end, has a rated power of 800MW and a dynamic reactive power capacity of ±150Mvar. Converter station B is connected to converter station D via DC line 3 (±500kV).

[0146] Converter station C, as the receiving end, has a rated power of 1200MVA and a dynamic reactive power capacity of ±250Mvar. Converter station C is connected to converter station A via DC line 2 (±500kV) and is ultimately connected to the AC power grid (Region 1), which has a voltage of 230kV and a short-circuit capacity of 6000MVA.

[0147] Converter station D, acting as the receiving end, has a rated power of 600 MVA and a dynamic reactive power capacity of ±100 Mvar. Converter station D is connected to converter station B via DC line 3 and ultimately connected to the AC power grid (Region 2), which has a voltage of 230 kV and a short-circuit capacity of 4000 MVA. S302: To calculate the improvement capability for the stability of a multi-loop DC system, the Effective Coordination Strength Ratio (MCESSR) is proposed. This ratio is used to quantify the comprehensive matching relationship between AC system strength, dynamic reactive power coordination capability, and redundancy support in a multi-loop system. It is defined as:

[0148]

[0149] : AC bus short-circuit capacity (MVA) at the connection point of the i-th converter station;

[0150] : Dynamic reactive power compensation capability (Mvar) of the i-th converter station;

[0151] : The reactive power contribution efficiency coefficient of the i-th converter station (0.5~1.0, determined by the control response speed);

[0152] : Rated transmission power (MW) of i loops;

[0153] Redundant power coordination term This refers to the system's transferable redundant power (MW). The coordination factor is 0.2~0.5, reflecting the power transfer efficiency during a fault.

[0154] The physical meaning of the effective synergistic strength ratio (MCESSR) is as follows:

[0155] MCESSR > 2.0: High system coordination strength, sufficient dynamic support between multiple loops, and strong fault tolerance capability;

[0156] 1.2 ≤ MCESSR ≤ 2.0: The coordination strength is moderate, and stability needs to be maintained by optimizing the control strategy;

[0157] MCESSR < 1.2: Insufficient coordination strength, with the risk of cascading failures or power blocking.

[0158] Furthermore, P red This represents the power capacity (overload capacity + reserve capacity) that can be transferred from the remaining circuit in the event of a fault.

[0159] 1) When a circuit fails, the remaining circuits take over the load of the failed circuit through power redistribution (e.g., increasing the overload by 20%).

[0160] 2) High MCESSR → P red Large → Transferable power ≥ 80% of fault power → Avoid load loss.

[0161] Dynamic reactive power coordination is the dynamic reactive power compensation capability of a converter station. i Quantify its response speed (0.5~1.0).

[0162] 1) When a fault causes a voltage drop, multiple converter stations simultaneously inject reactive power (e.g., STATCOM trips).

[0163] 2) High MCESSR → Larger → Faster voltage recovery (e.g., <300 ms) → Suppresses voltage collapse.

[0164] System strength support refers to the short-circuit capacity at the converter station connection point, reflecting the inherent strength of the AC power grid.

[0165] 1) High short-circuit capacity provides strong voltage support and reduces the risk of fault propagation.

[0166] 2) High MCESSR → Large → Small voltage fluctuation after a fault (e.g., drop ≤10%) → Avoids cascading tripping.

[0167] This invention proposes the Effective Coordination Strength Ratio (MCESSR) to quantify the comprehensive matching relationship between AC system strength, dynamic reactive power coordination capability, and redundancy support in multi-loop systems, and to verify stability. This not only improves the accuracy of stability assessment for multi-loop systems but also enhances the adequacy of dynamic support among multiple loops, thereby improving fault tolerance.

[0168] Furthermore,

[0169] S302-1: Perform data acquisition to obtain the short-circuit capacity of all converter stations. reactive power compensation capability Rated transmission power ; Calculate the maximum transferable power using N-1 security checks. (For example, the maximum capacity increase of other circuits during a certain fault). The calculation formula is as follows:

[0170]

[0171] Wherein, γ: converter station overload factor (usually taken as 1.2, i.e., 20% overload is allowed);

[0172] S sc,i : Short-circuit capacity (MVA) at the connection point of the i-th converter station;

[0173] Q dyn,i : Dynamic reactive power compensation capability (Mvar) of the i-th converter station;

[0174] k i Dynamic reactive power efficiency coefficient (0.5~1.0, determined by control response speed);

[0175] P dc,i : Rated transmission power (MW) of the i-th converter station.

[0176] For example:

[0177] Assume a DC transmission system containing three converter stations, with the following specific parameters:

[0178] Converter station 1: Ssc,1=3000MVA, Qdyn,1=200Mvar, k1=1.0 (fast response), Pdc,1=1000MW;

[0179] Converter station 2: Ssc,2=2500MVA, Qdyn,2=150Mvar, k2=0.8 (medium speed response), Pdc,2=800MW;

[0180] Converter station 3: Ssc,3=2000MVA, Qdyn,3=100Mvar, k3=0.5 (slow response), Pdc,3=600MW.

[0181] Assuming converter station 3 fails (j=3), calculate the Pred of the remaining converter stations 1 and 2.

[0182] Converter Station 1: = 200MW;

[0183] Converter Station 2: = 160MW.

[0184] S302-2: Calibrate the k value according to the effect of the control strategy: If the converter adopts fast voltage control (response time < 50ms), take k = 1.0; if it only has slow reactive power compensation (response time > 200ms), take k = 0.5.

[0185] Define the cooperating factor High coordination (e.g., DC grids have fast power routing): =0.5; Low coordination (e.g., independently controlled multiple circuits): =0.2.

[0186] S302-3: Calculate MCESSR;

[0187] S302-4: Perform stability verification by simulating simultaneous multi-loop faults (such as DC blockage in two loops) in PSCAD / EMTDC. Figure 7 As shown, observe the power increase and voltage stability of the remaining circuit, and observe the following indicators: After the fault, MCESSR still... 1.2, Voltage recovery time <300ms.

[0188] For example:

[0189] 1. MCESSR > 2.0: High system coordination strength

[0190] Parameter settings:

[0191] 1) Converter station 1: S sc,1 =4000MVA,Q dyn,1=300Mvar, k1=1.0 (fast response), P dc,1= 1000MW

[0192] 2) Converter station 2: S sc,2 =3500MVA,Q dyn,2 =250Mvar,k2=1.0,P dc,2 =1000MW

[0193] 3) Converter station 3: S sc,3 =3000MVA,Q dyn,3 =200Mvar,k3=1.0,P dc,3 =1000MW

[0194] 4) Redundant power: P red =1000MW, α=0.5 (high synergy)

[0195] ≈3.92

[0196] MCESSR = 3.92 > 2.0, indicating high system coordination strength, sufficient dynamic support between multiple loops, and strong fault tolerance.

[0197] 2. 1.2 ≤ MCESSR ≤ 2.0: Moderate synergy.

[0198] Parameter settings:

[0199] 1) Converter Station 1: S sc,1 =2000MVA, Q dyn,1 =200Mvar, k 1 = 0.8 (medium-speed response) P dc,1 =800MW

[0200] 2) Converter Station 2: S sc,2 =1800MVA, Q dyn,2 =150Mvar, k 2 = 0.8, P dc,2 =1200MW

[0201] 3) Redundant power: P red =500MW, α =0.3 (Low synergy)

[0202] ≈2.12

[0203] MCESSR = 2.12 ∈ [1.2, 2.0], with moderate cooperative strength, requiring optimization of control strategies to maintain stability.

[0204] 3. MCESSR < 1.2: Insufficient synergy.

[0205] Parameter settings:

[0206] 1) Converter Station 1: S sc,1 =1000MVA, Q dyn,1 =100Mvar, k 1 = 0.5 (slow response) P dc,1 =1500MW

[0207] 2) Converter Station 2: S sc,2 =800MVA, Q dyn,2 =50Mvar, k 2 = 0.5, P dc,2 =500MW

[0208] 3) Redundant power: P red =300MW, α =0.2 (Independent control)

[0209] ≈0.97

[0210] MCESSR = 0.97 < 1.2, indicating insufficient coordination strength and a risk of cascading failures or power blocking.

[0211] In another embodiment, the present invention provides a method for evaluating the effect of a controllable commutation converter on improving power system stability, such as... Figure 3 As shown:

[0212] First, determine whether the system contains a single-circuit DC branch or multiple-circuit DC branches: Calculate the total score for multiple-circuit characteristics based on the set weight thresholds (topology characteristics 40%, power characteristics 30%, fault response 30%). If the total score is ≥0.7, proceed to the "multi-circuit branch" branch; otherwise, proceed to the "single-circuit branch" branch.

[0213] Single-loop branch:

[0214] Data collection: Collect data such as short-circuit capacity, reactive power compensation capacity, and DC rated power;

[0215] Calibrate the k value: Based on the collected data, calibrate a coefficient k;

[0216] Calculate the effective system strength ratio (SCESRR);

[0217] If SCESRR > 1.0, the system is considered to have sufficient strength and strong stability improvement capability; otherwise, the system is considered to have insufficient strength and there is a risk of voltage collapse.

[0218] Multiple branching paths:

[0219] Data collection: Similarly, data such as short-circuit capacity, reactive power compensation capability, and rated transmission power are collected;

[0220] Calibrate the k value: Based on the collected data, calibrate a coefficient k;

[0221] Calculate the effective synergy ratio (MCESRR);

[0222] If MCESRR > 1.2, the system is considered to have high coordination strength, sufficient dynamic support between multiple loops, and strong fault tolerance; otherwise, the coordination strength is considered to be insufficient, and there is a risk of cascading failures or power blocking.

[0223] Figure 3 This paper presents a systematic method for evaluating the type and stability of DC systems. By calculating the weighted total characteristic score, single-circuit and multi-circuit systems can be accurately distinguished. Furthermore, by combining specific data collection and analysis, the stability and coordination capabilities of the system can be quantitatively assessed, thus providing a scientific basis for power grid planning and operation.

[0224] The above are merely preferred embodiments of this disclosure and are not intended to limit the implementation methods and protection scope of this disclosure. Those skilled in the art should recognize that any equivalent substitutions and obvious changes made using the content of this disclosure should be included within the protection scope of this disclosure.

[0225] 3) Redundant power: P red =500MW, α=0.3 (low synergy)

[0226] ≈2.12

[0227] MCESSR = 2.12 ∈ [1.2, 2.0], with moderate cooperative strength, requiring optimization of control strategies to maintain stability.

[0228] 3. MCESSR < 1.2: Insufficient synergy.

[0229] Parameter settings:

[0230] 1) Converter station 1: S sc,1 =1000MVA,Q dyn,1=100Mvar, k1=0.5 (slow response), P dc,1 =1500MW

[0231] 2) Converter station 2: S sc,2 =800MVA,Q dyn,2 =50Mvar,k2=0.5,P dc,2 =500MW

[0232] 3) Redundant power: P red =300MW, α=0.2 (independent control)

[0233] ≈0.97

[0234] MCESSR = 0.97 < 1.2, indicating insufficient coordination strength and a risk of cascading failures or power blocking.

[0235] In another embodiment, the present invention provides a method for evaluating the effect of a controllable commutation converter on improving power system stability, such as... Figure 3 As shown:

[0236] First, determine whether the system contains a single-circuit DC branch or multiple-circuit DC branches: Calculate the total score for multiple-circuit characteristics based on the set weight thresholds (topology characteristics 40%, power characteristics 30%, fault response 30%). If the total score is ≥0.7, proceed to the "multi-circuit branch" branch; otherwise, proceed to the "single-circuit branch" branch.

[0237] Single-loop branch:

[0238] Data collection: Collect data such as short-circuit capacity, reactive power compensation capacity, and DC rated power;

[0239] Calibrate the k value: Based on the collected data, calibrate a coefficient k;

[0240] Calculate the effective system strength ratio (SCESRR);

[0241] If SCESRR > 1.0, the system is considered to have sufficient strength and strong stability improvement capability; otherwise, the system is considered to have insufficient strength and there is a risk of voltage collapse.

[0242] Multiple branching paths:

[0243] Data collection: Similarly, data such as short-circuit capacity, reactive power compensation capability, and rated transmission power are collected;

[0244] Calibrate the k value: Based on the collected data, calibrate a coefficient k;

[0245] Calculate the effective synergy ratio (MCESRR);

[0246] If MCESRR > 1.2, the system is considered to have high coordination strength, sufficient dynamic support between multiple loops, and strong fault tolerance; otherwise, the coordination strength is considered to be insufficient, and there is a risk of cascading failures or power blocking.

[0247] Figure 3 This paper presents a systematic method for evaluating the type and stability of DC systems. By calculating the weighted total characteristic score, single-circuit and multi-circuit systems can be accurately distinguished. Furthermore, by combining specific data collection and analysis, the stability and coordination capabilities of the system can be quantitatively assessed, thus providing a scientific basis for power grid planning and operation.

[0248] The above are merely preferred embodiments of this disclosure and are not intended to limit the implementation methods and protection scope of this disclosure. Those skilled in the art should recognize that any equivalent substitutions and obvious changes made using the content of this disclosure should be included within the protection scope of this disclosure.

Claims

1. A method for evaluating the ability of a controllable commutator to improve power system stability, characterized in that, The method includes: S100. Determine whether the DC system in the power grid is a single-circuit DC system or a multi-circuit DC system; S200. Build a single-circuit DC power system model, propose an effective system strength ratio, and evaluate the stability improvement effect in the single-circuit DC system; S300. Build a multi-circuit DC power system model, propose an effective coordination strength ratio, and evaluate the stability improvement effect in the multi-circuit DC system; S400. Use simulation tools to verify the accuracy of the stability improvement effect; in, The effective system strength ratio (SCESSR) is calculated using the following formula: ， in, This refers to the short-circuit capacity of the receiving-end AC bus in a single-loop DC system. To enhance the dynamic reactive power compensation capability of the converter station, is the rated transmission power of the DC system, and k is the dynamic reactive power weighting coefficient; The effective synergy strength ratio (MCESSR) is calculated using the following formula: ， in, Let the short-circuit capacity of the AC bus at the connection point of the i-th converter station be . Let i be the dynamic reactive power compensation capability of the i-th converter station. Let be the reactive power contribution efficiency coefficient of the i-th converter station. Let i be the rated transmission power of the i loops. For redundant power coordination, This represents the system's transferable redundant power. As a cooperating factor; The effective system strength ratio SCESSR > 1.5 indicates sufficient system strength, high voltage stability, and strong fault recovery capability. 1.0 ≤ SCESSR ≤ 1.5: The system is at critical strength and requires dynamic control measures to maintain stability; SCESSR < 1.0: The system is not strong enough and there is a risk of voltage collapse or power transfer limitation. The effective coordination strength ratio MCESSR > 2.0 indicates that the system has high coordination strength, sufficient dynamic support between multiple loops, and strong fault tolerance. 1.2 ≤ MCESSR ≤ 2.0: The coordination strength is moderate, and stability needs to be maintained by optimizing the control strategy; MCESSR < 1.2: Insufficient coordination strength, with the risk of cascading failures or power blocking.

2. The method according to claim 1, characterized in that, S100 includes: Based on topology analysis, the nodes of the converter stations are obtained. If there are only 2 converter stations in the power grid and the DC lines are connected point-to-point, it is determined to be a single-circuit DC system. If there are ≥3 converter stations or multiple independent converter units are configured in the same converter station, it is determined to be a multi-circuit DC system.

3. The method according to claim 1, characterized in that, The S100 also includes: Monitor real-time data from each converter station, extract the DC network topology from the system or design documents, and establish a hybrid power flow model that includes DC. If all DC power is concentrated on a single line, it is determined to be a single-circuit DC system; if the power is distributed across multiple lines and has complementary characteristics, it is determined to be a multi-circuit DC system.

4. The method according to claim 1, characterized in that, The S100 also includes: If an N-1 fault is triggered by simulation or historical data, and the power increase of the remaining DC branch is greater than or equal to 80% of the capacity of the faulty branch, it is determined to be a multi-circuit DC system; otherwise, it is determined to be a single-circuit DC system.

5. The method according to claim 1, characterized in that, The S100 also includes: Set a weight threshold. If the total score of multiple features is ≥0.7, it is determined to be a multiple DC system; otherwise, it is determined to be a single DC system.

6. The method according to claim 1, characterized in that, The dynamic reactive power weighting coefficient k is set to 1.0 if the converter adopts fast voltage control, and 0.5 if it only has slow reactive power compensation.

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